Data communication systems make use of various channels such as telephone, radio or optical links to transmit digital data from one point to another. In order to establish route and conduct certain testing and problem diagnosis, certain control information has to be communicated between various terminals in the system. Terminal equipment at the receiving end of the channel must be prepared and notified to accept the data by means of control words, and the transmitting end of the link must be notified when the receiving station is ready to receive data and to signal it.
In the prior art, such control signals were transmitted over a separate link from that which was used to transmit the data, or was sent at a specified time known to both the transmitter and receiver so as to distinguish it from data. Such communication has often included voice transmission wherein the operators at both ends of the link would confirm that the link had been established and that the equipment was in readiness for the transmission. Such techniques require additional bandwidth in the form of a separate channel or time allocated within the data channel to distinguish control information from data. In synchronous links, differences in communications time caused by different signal paths between the two links require that sufficient time be allocated before data is transmitted to insure that the receiver has properly received the control signals. In either case, it is required to establish or maintain a separate control channel in order to regulate the transmission of data. Additional bandwidth in the form of a separate channel or in allocated times on the primary channel are referred to as overhead connected with the transmission of the data.